Re-throw an exception in a conditional expression

Question

What is the correct way of rewriting the code in the following catch block using a conditional expression? if supported!?

try { return await _client.GetStreamAsync(_uri); } catch { if (IsConnected) throw; else throw new IOException(); }

C# compiler does not like the following

IsConnected ? throw : new IOException();

Note that re-throwing a caught exception, like the following, is in violation of CA2200

try { return await _client.GetStreamAsync(_uri); } catch (Exception ex) { throw IsConnected ? throw ex : new IOException(); }

Are you getting an error message on the if version? Im pretty sure the ternary is not legal C#; ternaries are expressions, not statements — Flydog57
– Flydog57, Commented Dec 22, 2021 at 3:28
Do you mean the if in the first example? if so, no, that works fine. — Dr. Strangelove
– Dr. Strangelove, Commented Dec 22, 2021 at 3:30
@Flydog57 throw someException is legal in a ternary, but throw is not, this is from C#7 stackoverflow.com/questions/42209135/… — Charlieface
– Charlieface, Commented Dec 22, 2021 at 4:02

Dr. Strangelove · Accepted Answer · 2021-12-22 04:54:41Z

5

Your first example is fine. throw; will rethrow the exception leaving the stack trace in tact. As an alternative, I would suggest just conditionally catching the exception when IsConnected == false:

catch when (IsConnected) { throw new IOException(); }

You can read more about the when keyword in the docs.

edited Dec 22, 2021 at 4:54

Dr. Strangelove

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answered Dec 22, 2021 at 3:42

ProgrammingLlama

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5 Comments

DK. Over a year ago

Good catch.

Dr. Strangelove Over a year ago

Thanks, that makes sense. For sake of completeness, would it be correct to assume that your suggestion implies that re-throwing an exception in the pattern I wrote in the first block, is not possible in a conditional expression?

ProgrammingLlama Over a year ago

@Hamed No, you certainly can rethrow it. For example: catch (Exception e) when (someCondition) { throw; } would work fine, as would more complicated logic in the catch.

Dr. Strangelove Over a year ago

Actually, I meant in a pattern like condition ? throw : anotherException(). Though I agree your suggested approach is more clear/readble.

ProgrammingLlama Over a year ago

I believe the throw part alone in a ternary expression isn't valid C#.

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